POPULAE EXPLANATION. 23 



is never found beneath limestone. It may be found either- 

 above or beneath it, yet there is very little limestone, and 

 that quite impure, in the productive coal-measures of Iowa; 

 but, as will be seen by referring to the map-model before 

 described, a formation composed principally of limestone 

 (sub-carboniferous) underlies the productive coal-measures 

 (the Lower and Middle) and another, also principally of 

 limestone (the Upper coal-measures) overlies those coal-bear- 

 ing formations. Beneath the first named limestone formation 

 it is of course useless to search for coal, as that formation is 

 already beneath all the coal-bearing strata, while we may 

 reasonable expect to reach those last named strata if we 

 pierce the upper formation of limestone represented in the 

 map-model by the sheet marked " Upper Coal-Measures." 



As before remarked, there are, strictly speaking, no surface 

 indications that absolutely prove the presence of coal beneath 

 any given spot, but since coal-beds are known to have very 

 considerable extent beneath the surface, and to lie in a nearly 

 uniform plane, irrespective of the inequalities of the surface, 

 one may in certain cases make very accurate determinations 

 as to its presence and position beneath the surface before 

 digging down to it. This is not accomplished by observing 

 any occult " signs," "indications," or "blossoms," but by 

 legitimate deductions from facts personally known. Thus, if 

 a bed of coal is known to crop out on one side of a valley, 

 and the strata associated with it are known to be level or 

 nearly so, we may expect to find the same bed at about 

 the same level upon the other side. If a similar exposiu*e 

 should be found in another valley a few miles distant, it is 

 reasonable to infer, that the whole space between the two 

 valleys is underlaid by the same continuous bed of coal, and 

 that it may be reached from the surface at a depth which 

 may be calculated with considerable accuracy. Besides this, 

 it will be seen from what has before been explained, that by 

 a careful study of all the rocks of any given region the 

 geologist may determine the limits of any formation, and if 

 it is a coal-bearing one, he is able to indicate a line upon the 



